1. Field of the Invention
This invention relates to a double-hetero structure semiconductor diode laser device. More particularly, the invention relates to a structure of a semiconductor laser device having a stabilized lasing mode and a high reliability.
2. Description of the Prior Art
Semiconductor laser devices are now indispensable elements as light sources in optical communication systems. As is well known in the art, a double hetero (DH) structure is adopted for semiconductor devices which operate effectively at room temperature. A typical structure of the most advanced prior art GaAs-GaAlAs double hetero semiconductor laser device is illustrated in FIGS. 1-a and 1-b. FIG. 1a is a view showing the section of the element taken along a direction parallel to light, and FIG. 1-b is a view showing the section of the element of FIG. 1-a taken along a direction perpendicular to light. Such an element is ordinarily prepared by growing on an n-GaAs substrate 1 a layer 2 of n-Ga.sub.1-x Al.sub.x As (for example, x is 0.3) corresponding to a second semiconductor, a layer 3 of p-GaAs corresponding to a first semiconductor, a layer 4 of p-Ga.sub.1-x Al.sub.x As (for example, x is about 0.3) corresponding to a third semiconductor and a layer 5 of p-GaAs facilitating electric connection to a positive electrode 6 by successive liquid phase epitaxy, then forming electrodes and cleaving crystals to form a reflection face 8. Reference numeral 7 represents a negative electrode. When this laser device is driven at 2.2 V and 100 mA (2 KA/cm.sup.2) at room temperature, a continuous light output power of .about.8900 A and .about. 10 mW under CW conditions is obtained. The laser device having the above structure can easily be prepared in a high production yield and it has a long life and a high reliability. However, a laser device of this type is defective in that the lasing mode becomes unstable. More specifically, in the above structure, the region of the first semiconductor layer 3 below the positive electrode 6 has a lasing action, but since the refractive index is not intentionally changed in the direction x in this layer, the lasing mode in the direction x is determined by a slight refractive index profile and gain profile generated by application of an electric current. Such a refractive index profile or gain profile is remarkably changed depending on changes in the excitation current or temperature and on configurations of the element such as the layer thickness. Accordingly, in general, the lasing mode shows very irregular changes and has no reproducibility. This unstability of the transverse mode has bad influences on the linearity of the excitation current or light output power. When modulation is conducted by a pulsating current, unstable variations are caused in the light output power, the signal-to-noise ratio is degraded, and the directivity of the output light is rendered unstable. Therefore, it becomes difficult to introduce the light output power at a high efficiency stably to other optical systems such as light fibers. Thus, practical use of such laser device involves various problems.
Some attempts have heretofore been made to eliminate the foregoing disadvantages. For example, a so-called BH (buried hetero structure) laser device has been developed [T. Tsudaka, J. Appl. Phys., 45, 4897 (1975)]. A sectional view of this laser device is illustrated in FIG. 2. Referring now to FIG. 2, a p-GaAs layer 3 which is a region having a lasing action is surrounded by an n-Ga.sub.1-x Al.sub.x As layer 2, a p-Ga.sub.1-x Al.sub.x As layer 4 and an n-Ga.sub.1-x Al.sub.x As layer 9, each having a lower refractive index than that of the layer 9. In this arrangement, a definite change of the refractive index is present also in the direction x. Accordingly, the transverse mode is stabilized, and characteristic difficulties involved in the element structure shown in FIGS. 1-a and 1-b are eliminated. In order to realize the element structure shown in FIG. 2, an n-Ga.sub.1-x Al.sub.x As layer 2, a p-GaAs layer 3 and a p-Ga.sub.1-x Al.sub.x As layer 4 are grown on an n-GaAs substrate 1 by successive liquid phase epitaxy, and then, the structure is processed into a mesa-stripe form and an n-Ga.sub.1-x Al.sub.x As is grown by liquid phase epitaxy. Accordingly, the production steps are complicated and the production yield is very low. In addition to these defects, there is another defect that during the production process, especially at the regrowth step, crystal defects are readily caused and have bad influences on factors concerning the practical utility, such as life and reliability.
A spot-like laser device capable of lasing in the single transverse mode is applicable as a light source for single mode fiber communication or a light source for light information processing devices such as a video disk, and development of a laser device meeting requirements in this application field is desired in the art. In this application field, it generally is required that the width of spots of the light output power should be between about 1 to about 8 .mu.m.
As the laser device in which a single transverse mode operation is obtained in the junction face of the laser, there can be mentioned a buried hetero structure laser device as illustrated in FIG. 2 and a transverse-junction stripe laser device (IEEE Journal of Quantum Electronics, Vol. QE-11, No. 7, July, 1975). The former device is defective in that the spot width is limited to about 1 .mu.m or less and growth of crystals must be conducted 2 times. The latter device is defective in that zinc must be diffused deeply even into the lasing active region after growth of crystals and, therefore, a complete reliability cannot be attained.